(A) Field of the Invention
The present invention is related to an over-current protection device, more specifically, to an over-current protection device for low load applications.
(B) Description of the Related Art
Because the resistance of positive temperature coefficient (PTC) conductive composite material is sensitive to temperature variation, it can be used for current sensing devices and is widely used for over-current protection devices or circuits. The resistance of PTC conductive composite materials can be kept extremely low at room temperature so that the circuit can operate normally. However, if an over-current or an over-temperature event occurs, the resistance will immediately increase to a high resistance state (e.g., above 104Ω.) Therefore, the over-current will be eliminated and the objective to protect the circuit device will be achieved.
Generally, PTC conductive composite material comprises at least one crystalline polymer and conductive filler. The conductive filler is uniformly distributed into the polymer. The polymer can be polyolefin polymer, e.g., polyethylene, and carbon black is in wide use as the conductive filler. In the past, portable electronic devices are relatively large in size. The PTC used in the battery for the electronic device is also large in size. As the technology advance in recent year, the portable electronic devices are getting smaller, lighter, and more functions. The devices demand much more current in use and longer service life. However, due to the high volume resistivity (>0.2Ω-cm) of carbon black, the carbon black loaded PTC device consumes too much energy and shortens the service life. Therefore, it is much desirable to have over current protection PTC device with volume resistivity less than 0.1 Ω-cm.
Because providing protection at low temperature is necessary for the over-current protection to a battery, the PTC conductive composite material usually uses polymer of a low melting temperature as matrix thereof, e.g., low density polyethylene (LDPE). As a result, the trip temperature is reached at a relatively low temperature, so that explosion of or damage to the battery due to over-temperature can be avoided.
However, if the PTC conductive composite material comprising LDPE matrix is used for a long time, the resistance thereof will increase gradually. For example, if it is subjected to a thermal shock between −40° C. and +85° C. for 100 cycles, the resistance thereof will increase from 10 mΩ initially to above 1Ω. The resistance cannot return to the initial value, so the device is not suitable for low resistance electric apparatus such as a battery.
As is well known, adding HDPE to the LDPE matrix can solve the above resistance drift problem. LDPE and HDPE, however, may form a compatible polymer blend which means that LDPE and HDPE could be partially dissolved into each other. Due to the presence of high crystalline HDPE, the melting point of the polymer blend could be dominated by the HDPE when HDPE exceeds 25% of the polymer weight. Accordingly, the trip temperature of LDPE with HDPE added is obviously higher than that of pure LDPE as shown in FIG. 1. In other words, the trip temperature of the PTC conductive composite material is increased, thus protection at low temperatures cannot be achieved. Therefore, if it is used in lithium ion batteries, explosion or burning of the batteries may occur.
In view of the above, there is a tradeoff between increasing the resistance repeatability and decreasing trip temperature. There remains a need for a breakthrough for low load applications.